The subject matter described herein relates generally to gas turbine engines, and more specifically, to turbine engines having a low fan pressure ratio shrouded fan with a pitch change mechanism for increasing the efficiency of the gas turbine engine.
Known turbofan gas turbine engines used for powering an aircraft in flight typically include a fan assembly, a low pressure compressor, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The combustor generates combustion gases that are channeled to the high pressure turbine where they are expanded, driving the high pressure turbine, and then to the low pressure turbine where they are further expanded, driving the low pressure turbine. The high pressure turbine is coupled to the high pressure compressor by a high speed rotor shaft, and the low pressure turbine is coupled to both the fan assembly and the low pressure compressor by a low speed rotor shaft.
To increase efficiency of such known turbine engines, a large diameter fan assembly is desirable, which enables a higher bypass ratio to be used. However, the size of the fan assembly is dependent on the size of the turbine blades that drive the fan assembly. As the fan assembly size increases, the turbine size generally must also increase to maintain high efficiency. Furthermore, high bypass ratio turbofans suffer from large weight and drag penalties associated with their very large fan ducts. Moreover, the long fan blades on large fans are susceptible to flutter and inlet flow distortion, which results in unsteady loading of the fan blades.